Subject: Re: [PATCH v5 1/5] arm64: Add framework to turn IPI as NMI

From: Marc Zyngier <[email-masked]>
Date: Mon, 19 Oct 2020 12:37:47 +0100

On 2020-10-14 12:12, Sumit Garg wrote:
> Introduce framework to turn an IPI as NMI using pseudo NMIs. In case a
> particular platform doesn't support pseudo NMIs, then request IPI as a
> regular IRQ.
> 
> The main motivation for this feature is to have an IPI that can be
> leveraged to invoke NMI functions on other CPUs. And current 
> prospective
> users are NMI backtrace and KGDB CPUs round-up whose support is added
> via future patches.
> 
> Signed-off-by: Sumit Garg <[email-masked]>
> ---
>  arch/arm64/include/asm/nmi.h | 16 +++++++++
>  arch/arm64/kernel/Makefile   |  2 +-
>  arch/arm64/kernel/ipi_nmi.c  | 77 
> ++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 94 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/include/asm/nmi.h
>  create mode 100644 arch/arm64/kernel/ipi_nmi.c
> 
> diff --git a/arch/arm64/include/asm/nmi.h 
> b/arch/arm64/include/asm/nmi.h
> new file mode 100644
> index 0000000..3433c55
> --- /dev/null
> +++ b/arch/arm64/include/asm/nmi.h
> @@ -0,0 +1,16 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_NMI_H
> +#define __ASM_NMI_H
> +
> +#ifndef __ASSEMBLER__
> +
> +#include <linux/cpumask.h>
> +
> +extern void arch_send_call_nmi_func_ipi_mask(cpumask_t *mask);
> +
> +void set_smp_ipi_nmi(int ipi);
> +void ipi_nmi_setup(int cpu);
> +void ipi_nmi_teardown(int cpu);
> +
> +#endif /* !__ASSEMBLER__ */
> +#endif
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index bbaf0bc..525a1e0 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -17,7 +17,7 @@ obj-y			:= debug-monitors.o entry.o irq.o fpsimd.o		\
>  			   return_address.o cpuinfo.o cpu_errata.o		\
>  			   cpufeature.o alternative.o cacheinfo.o		\
>  			   smp.o smp_spin_table.o topology.o smccc-call.o	\
> -			   syscall.o proton-pack.o
> +			   syscall.o proton-pack.o ipi_nmi.o
> 
>  targets			+= efi-entry.o
> 
> diff --git a/arch/arm64/kernel/ipi_nmi.c b/arch/arm64/kernel/ipi_nmi.c
> new file mode 100644
> index 0000000..a959256
> --- /dev/null
> +++ b/arch/arm64/kernel/ipi_nmi.c
> @@ -0,0 +1,77 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * NMI support for IPIs
> + *
> + * Copyright (C) 2020 Linaro Limited
> + * Author: Sumit Garg <[email-masked]>
> + */
> +
> +#include <linux/interrupt.h>
> +#include <linux/irq.h>
> +#include <linux/smp.h>
> +
> +#include <asm/nmi.h>
> +
> +static struct irq_desc *ipi_desc __read_mostly;
> +static int ipi_id __read_mostly;
> +static bool is_nmi __read_mostly;
> +
> +void arch_send_call_nmi_func_ipi_mask(cpumask_t *mask)
> +{
> +	if (WARN_ON_ONCE(!ipi_desc))
> +		return;
> +
> +	__ipi_send_mask(ipi_desc, mask);
> +}
> +
> +static irqreturn_t ipi_nmi_handler(int irq, void *data)
> +{
> +	/* nop, NMI handlers for special features can be added here. */
> +
> +	return IRQ_HANDLED;

This definitely is the *wrong* default. If nothing is explicitly
handling the interrupt, it should be reported as such to the core
code to be disabled if this happens too often.

> +}
> +
> +void ipi_nmi_setup(int cpu)

The naming is awful. "ipi" is nowhere specific enough (we already have
another 7 of them), and this doesn't necessarily uses pseudo-NMIs, since
you are requesting IRQs.

> +{
> +	if (!ipi_desc)
> +		return;
> +
> +	if (is_nmi) {
> +		if (!prepare_percpu_nmi(ipi_id))
> +			enable_percpu_nmi(ipi_id, IRQ_TYPE_NONE);
> +	} else {
> +		enable_percpu_irq(ipi_id, IRQ_TYPE_NONE);

I'm not keen on this. Normal IRQs can't reliably work, so why do you
even bother with this?

> +	}
> +}
> +
> +void ipi_nmi_teardown(int cpu)
> +{
> +	if (!ipi_desc)
> +		return;
> +
> +	if (is_nmi) {
> +		disable_percpu_nmi(ipi_id);
> +		teardown_percpu_nmi(ipi_id);
> +	} else {
> +		disable_percpu_irq(ipi_id);
> +	}
> +}
> +
> +void __init set_smp_ipi_nmi(int ipi)
> +{
> +	int err;
> +
> +	err = request_percpu_nmi(ipi, ipi_nmi_handler, "IPI", &cpu_number);
> +	if (err) {
> +		err = request_percpu_irq(ipi, ipi_nmi_handler, "IPI",
> +					 &cpu_number);
> +		WARN_ON(err);
> +		is_nmi = false;
> +	} else {
> +		is_nmi = true;
> +	}
> +
> +	ipi_desc = irq_to_desc(ipi);
> +	irq_set_status_flags(ipi, IRQ_HIDDEN);
> +	ipi_id = ipi;

Updating global state without checking for errors doesn't seem
like a great move.

         M.
-- 
Jazz is not dead. It just smells funny...

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